Plumb CompositingReason from Blink to compositor.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index f1ab5429df4b19c8c4d54e5d53024486c50f8fbe..680124545ffeb6b5f650ec351d420f4e0235943c 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -116,7 +116,16 @@ enum {
 
     // The root layer is a special case that may be forced to be a layer, but also it needs to be
     // a layer if anything else in the subtree is composited.
-    CompositingReasonRoot                                   = 1 << 25
+    CompositingReasonRoot                                   = 1 << 25,
+
+    // RenderLayerBacking internal hierarchy reasons
+    CompositingReasonLayerForClip                           = 1 << 26,
+    CompositingReasonLayerForDescendantClip                 = 1 << 27,
+    CompositingReasonLayerForScrollbar                      = 1 << 28,
+    CompositingReasonsLayerForScrollingContainer            = 1 << 29,
+    CompositingReasonsLayerForForeground                    = 1 << 30
+
+    // Note: if you add more reasons here, you will need to update the WebCompositingReasons enum as well.
 };
 typedef unsigned CompositingReasons;